In the context of the present invention, the expression “coloring material” denotes a substance which acts as dye in the medium into which it has been introduced or on the surface of the material or support to which it has been applied.
In the cosmetics field, use is very often made of coloring materials of natural or synthetic origin and of inorganic or organic chemical nature. They are used mainly to produce a coloring effect on the skin or superficial body growths, as is the case for makeup products. These coloring materials can, in particular, be of plant origin.
Plant dyes are transparent in a liquid medium. Mention is made, among those which can be used in cosmetics, of anthocyanins, carotenoids, curcumin, and chlorophylls. They can be used to simply color a composition, without any coloring effect on the skin or superficial body growths. However, if it is desired to use them in makeup products or more generally to color a surface, in order for the rendering of the colors on the skin or superficial body growths or the surface to be satisfactory, it is necessary to render them opaque. For this, they are rendered insoluble. The main technique for rendering plant dyes insoluble consists in fixing them to a solid substrate.
The coloring materials obtained in the solid form include “lakes”. Usually composed of three elements: a dye, a substrate, for example alumina, and a cation which acts as precipitating agent, lakes are used in particular in the preparation of paints, transparent coatings and glazes. These lakes are also sometimes used to color food preparations but more rarely in cosmetic compositions.
The lakes of natural dyes which are the most widely used have been manufactured starting from various anthraquinone-comprising plants of the family of the Rubiaceae, an example of which is provided by common madder (Rubia tinctorum L.), from some anthraquinone-rich insects or from a few other plants rich in flavonoids, neoflavonoids and other dyes of the same type (yellow lakes of mignonette and various Moraceae, purple lakes of logwood, pink and red lakes of woods from various members of the Caesalpiniaceae, and the like).
The conventional process for manufacturing a natural dye lake consists in bringing together a colored or uncolored aqueous extract originating from a vegetable, mineral or animal source and a metal salt, for example an aluminum salt in the form of a sulfate or of a chloride, in order to form a colored organometallic complex, and in precipitating said complex thus formed by addition of a cation in the form, for example, of a sodium, potassium or calcium salt.
Within the meaning of the invention, a “lake” is thus defined as a colored solid formed by precipitation, using a cation, of a colored organometallic complex formed in particular by addition of a metal cation to an aqueous extract.
“Complex” is understood to mean, in the present description, the product resulting from the combination of a metal cation with a ligand of organic nature.
The coloring of the complex may result from the use of a colored aqueous extract or else may appear only at the time of the complexing reaction with the metal cation. This coloring can vary according to the metal cation chosen for the preparation of the complex.
The molecules capable of forming colored organometallic complexes include the molecules belonging to the family of the tannins.
The tanning industry makes use of their natural affinity for various metal salts in preparing leather and hides (leather with tannin retreated with magnesium salts in particular).
Tannin/iron complexes have also been used since time immemorial in the form of fluid inks or for various applications in the field of the dyeing of textiles.
Tannins are found in most plants and in all their parts, in particular the bark, roots or leaves.
These tannins are polyphenolic acid derivatives which result from the esterification by these acids of the alcohol functional groups of sugars. They have a highly variable chemical structure but one which always comprises a polyphenolic part.
Standing out among the tannins are hydrolysable tannins, which are esters formed by reaction of a natural catechic acid, such as gallic acid, caffeic acid or ferulic acid, or of one of their oligomers or polymers, such as pentagalloylgallic, dehydrodigallic, ellagic, chebulic, hexahydroxydiphenic, nonahydroxytriphenic, valoneic, sanguisorbic, trilloic or gallagic acid, with a natural alcohol, generally a polyol, such as sugars (glucose, rhamnose, rutinose, fructose, galactose, mannose), inositol, quinic acid or shikimic acid.
These hydrolysable tannins can be:                monoesters, such as glucogallin, theogallin or chlorogenic acid;        polyesters, such as pentagalloylglucose, hexagalloylglucose, geranins, geranamines, tanic acids, tellimagrandins, casuarictin or pedunculagin;        oligomers of these polyesters, such as gemins, rugosins, isorugosins, cornusiins, coriarins, oenotheins, agrimonin, sanguins, corilagin, granantins and the other hydrolysable tannins isolated by the authors cited in the work “Plant Polyphenols: Vegetable Tannins Revisited”, Haslam E., published by CUP Archive, in particular chapter 3, pages 90-153.        
Mention is in particular made, among hydrolysable tannins, by way of example and without being limiting, of:                gallic tannins (or gallotannins), which are esters of gallic acid and of digallic acid with C5 or C6 sugars, such as glucose, or derivatives, such as hamamelose, ribose derivatives,        ellagic tannins (or ellagitannins), which are esters of ellagic acid.        
Hydrolysable tannins are not naturally colored but result in colored substances by the processes already explained, particularly with iron, with which they form a complex which is black in color.
However, dye lakes based on organometallic complexes comprising hydrolysable tannins are not used to any great extent as they exhibit a mediocre stability. In particular, lakes of gallic tannins and of iron have not been developed, in particular because the tannins/iron complex with a black color is very difficult to precipitate and at the same time very unstable, with a tendency to redissolve. Reference is made, in this case, to a “delaking” phenomenon.
In order to be in a position to use a dye lake based on hydrolysable tannins and metal salts in compositions, in particular cosmetic compositions, especially in makeup products, it is essential to stabilize it in order to prevent a detrimental change in the color over time, by a delaking phenomenon, and a detrimental change in the intrinsic properties of these compositions.
To date, there does not exist any stabilized lake based on hydrolysable tannins, neither does there exist a solid coloring material manufactured from plant extracts comprising such hydrolysable tannins, the stability of which allows them to be used as a formulation ingredient.
It is precisely the problem of the stabilization of dye lakes based on hydrolysable tannins and metal cations which the present invention intends to solve. Within the meaning of the invention, a dye lake is stable when it does not exhibit any delaking phenomenon, that is to say any dissolution of the colored complex when the lake is suspended in a solvent normal in the field of application, or detrimental change in color, in particular after prolonged exposure to a light source.